A very wide variety of vibration isolators exist, the most familiar being the common rubber mount. A typical passive vibration mount uses a rubber pad operating as a mechanical spring. This pad is positioned between a platform and a base. The pad may be configured and dimensioned in various manners, however its principle purpose is to provide a spring-type element that supports the platform. The system then mechanically behaves as a spring-mass oscillator, where the mass M is largely that of the platform. Such a mechanical system has a fundamental resonance frequency fr. This resonance is related to the mass M and the spring constant ke by
      f    r    =            1              2        ⁢                                  ⁢        π              ⁢                            k          e                M            when there is little or no damping present.
At frequencies below the resonance frequency, the mechanical motions of the platform and base are strongly coupled, such that a motion of one causes a similar motion in the other. However at frequencies above this resonance, the two become lightly coupled, and the mechanical motions or vibrations in one do not strongly affect the other. The degree of isolation typically improves as the ratio of the test frequency to the resonance frequency increases. For example, for the simple isolator system described above, the transmissibility that describes the platform motion Δx resulting from a base support motion
      Δ    ⁢                  ⁢          x      0        ⁢                  ⁢    is    ⁢                  ⁢                  Δ        ⁢                                  ⁢        x                    Δ        ⁢                                  ⁢                  x          0                      =            1              1        -                  (                      fr            /            f                    )                      .  
FIG. 1 illustrates characteristics of a typical passive rubber isolator. High isolation is achieved at frequencies much higher than the resonance frequency of the system. As a result, high performance over a desired frequency band requires that the passive isolation mount have a very low resonant frequency. Simple versions are often designed to isolate vibrations along the just vertical axis, however higher performance configurations isolate along all three axes. Commercially available units of this type are commonly available for a wide variety of applications.
This and other such passive isolators perform well in many applications, but have disadvantages that limit their range of applicability. The passive vibration mount has several limitations. The most serious is that very high performance is difficult to achieve. For this type of isolator to have high performance, the resonance frequency must be very low. This requires a very soft isolator, which can lead to platform stability problems, since there is little restoring force restricting the off-axis motions of the platform.
In active vibration isolators, a fast-acting motor, called an actuator, replaces the rubber element. Sensors are located on the platform and/or base to monitor motion or vibration. The output of these sensors (typically accelerometers) is an electrical signal, which is conditioned by a control system, amplified and applied to the actuator. The desired result is that the actuator motion reduces the dynamic mechanical coupling between the platform and base over the frequency band of interest.
Some hybrid isolation systems have added an active control system to an existing passive rubber isolator. Such hybrid systems have been demonstrated for specific single-axis laboratory test structures.